Optical spectropolarimetry of large C-complex asteroids: Polarimetric evidence for heterogeneous surface compositions

This study presents the first optical (420–930 nm) spectropolarimetric study of a sample of large (≳100 km in diameter) C-complex asteroids in observing circumstances for which light scattered by asteroid surfaces undergoes a so-called negative polarization. We analyzed a total of 64 C-complex asteroids of different subclasses, using archival polarimetric and reflectance data to refine the link between polarimetric parameters and surface properties of the asteroids. We find a consistent difference in the polarization spectra between asteroids containing phyllosilicates and those without, which is correlated with the overall morphology of the reflectance spectrum. The inverse relationship between reflectance and the polarization degree, known as the Umov law, is not very sharp in our sample of asteroids. These objects exhibit broad similarities in polarization-phase curves; nonetheless, we do observe a gradual enhancement of the negative polarization branch (both in depth and width) in the ascending order of F → B → T → Ch types (Spearman correlation coefficient ρ = 0.70 -0.07 +0.06 and p-value p = 0.0001), along with an increase in the reflectance curvature around 500 nm. A weaker inverse correlation between the inversion angle and diameter of the asteroids in our sample has been found with ρ = -0.42 -0.07 +0.08 and p = 0.01, apparently driven by the distribution of C-type asteroids ( ρ = -0.51 -0.11 +0.13 and p = 0.04). Our observations suggest that at least for large C-complex asteroids: 1) a common mechanism underlies the diversity in polarimetric and spectroscopic properties at optical wavelengths; 2) the observed trends would be explained by the surface composition of the asteroids, particularly with respect to the optical heterogeneity on the wavelength scale caused by carbon’s varying levels of optical influence; and 3) aqueous alteration may play a significant role in regulating this operational effect of carbon.